minor #14734 [Session] Update incorrect default value for session.handler_id (Nyholm)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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[Session] Update incorrect default value for session.handler_id

See config [definition](https://github.com/symfony/symfony/blob/4.4/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php#L557) and [how it is being used](https://github.com/symfony/symfony/blob/4.4/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php#L949).

Commits
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5570ab8 [Session] Update incorrect default value for session.handler_id

Author: javiereguiluz

reference/configuration/framework.rst

@@ -1257,11 +1257,11 @@ alias will be set to this service id. This class has to implement
 handler_id
 ..........
 
-**type**: ``string`` **default**: ``null``
+**type**: ``string`` **default**: ``'session.handler.native_file'``
 
-The service id used for session storage. The default ``null`` value means to use
-the native PHP session mechanism. Set it to ``'session.handler.native_file'`` to
-let Symfony manage the sessions itself using files to store the session metadata.
+The service id used for session storage. The default value ``'session.handler.native_file'``
+will let Symfony manage the sessions itself using files to store the session metadata.
+Set it to ``null`` to use the native PHP session mechanism.
 You can also :doc:`store sessions in a database </session/database>`.
 
 .. _name: